Have you ever observed that you are not getting full speed while downloading something. Well, the reason is when you start a download from a browser, your browser usually opens a single stream to download the file. So even if you have 10Mbps connection, but the file server has limited the speed to 1Mbps per stream, your download speed will be limited to 1Mbps. But if you use a good download manager such as Internet Download Manager, it tries to download streams of the file in parallel, so the download is faster.